1. Field of the Invention
The present invention relates to X-ray examination devices, and in particular to such devices having at least two beam collimators in the form of slit diaphragms, one of which is disposed close to the focus of the X-ray tube and the other of which is disposed close to the X-ray film on which the radiation is incident.
2. Description of the Prior Art
The level of contrast and resolution of X-ray photographs, both of which contribute to the recognition of details in such photographs, are known to be increased by reducing the stray radiation component of the X-ray beam. For this purpose, a stray radiation screen disposed directly in front of the image plane of the radiation detector in the radiation direction is generally utilized for reducing the stray radiation component. Another commonly utilized method for reducing the stray radiation component is to collimate the radiation cone by utilizing only a solid angle portion thereof defined by a slit diaphragm. This segment of the X-ray beam is swept over the examination subject, and the X-ray film cassette disposed below the patient, and instead of utilizing a stray radiation screen, a second slit diaphragm which also collimates the fan-shaped radiation beam is synchronously moved in alignment with the first slit diaphragm across the X-ray film cassette. By means of this so-called "double-slit diaphragm method" not only is less stray radiation generated per time unit in the volume of the examination subject (this being a direct consequence of the reduction of the irradiated volume) but also a negligibly small portion of the stray radiation which is present strikes the X-ray film due to the presence of the second slit diaphragm. In exchange for these advantages, however, the equipment outlay for the synchronous control of the two slit diaphragms is increased and longer exposure times with a corresponding higher X-ray tube load are required.
A known X-ray examination device for generating X-ray photographs according to the double slit diaphragm method is described in U.S. Pat. No. 4,097,748. In this X-ray examination device, the fan-shaped radiation beam segment of the X-ray beam cone is conducted across the examination area perpendicularly with respect to the plane of the fan. The second beam collimator, that is, the second slit diaphragm, is disposed in the radiation direction directly in front of the X-ray film plane and is moved across the film plane synchronously with the fan shaped radiation beam in alignment therewith. To this end, the motor which adjusts the position of the X-ray tube (and the first slit diaphragm attached thereto) and the other motor which displaces the second slit diaphragm disposed above the film plane are controlled in such a manner that the fan-shaped radiation beam is always in registry with those two slit diaphragms. A characteristic of this design is that the outlay for the synchronous movement of the two slit diaphragms considerably increases as the diaphragm slit width becomes smaller because of the increased demands made on the precision of the control means. Recognition of detail on the X-ray photograph, however, improves as the slit diaphragm (and the fan-shaped radiation beam) is made narrower. This known device has the further disadvantage that a user of this type of X-ray device who wishes to retain the capability of conducting an X-ray examination using the entire radiation cone with a stray radiation screen, instead of the double slit method, must either purchase two X-ray devices or make a choice between the full beam method and the double slit method.